A methanation reaction comprises a catalytic reaction of hydrogen with carbon monoxide to produce methane. This methane is sometimes also referred to as synthetic natural gas (SNG) and can be used as substitute gas for natural gas. In areas where there is little natural gas available, other sources of energy, such as coal or petroleum coke, may be partially oxidized in a gasification process to produce a gas comprising hydrogen and carbon monoxide. Such a gas comprising hydrogen and carbon monoxide is sometimes also referred to as synthesis gas. The synthesis gas can subsequently be used to produce synthetic natural gas (SNG) in a methanation process.
The methanation reaction proceeds, in the presence of a suitable methanation catalyst, in accordance with the following equations:CO+3H2=CH4+H2O+heat  (1)CO2+4H2=CH4+2H2O+heat  (2).
The water formed during the reaction can, depending on the catalyst, temperature and concentrations present, subsequently react in-situ with carbon monoxide in a water-gas shift reaction in accordance with the following equation:CO+H2O=CO2+H2+heat  (3)
Reaction (1) is considered the main reaction and reactions (2) and (3) are considered to be side reactions. All the reactions are exothermic.
The methanation reactions are reversible and an increasing temperature will tend to shift the equilibrium towards a lower yield. To control the temperature, the methanation reaction can be carried out in one or more internally cooled reactors, where the reactants can be cooled by a coolant.
U.S. Pat. No. 4,839,391 describes a one-stage process for the methanation of synthesis gas to generate methane and superheated steam. The methanation reactor comprises a catalyst bed with different temperature regions, through which a cooling system passes. A cooling medium, i.e. water, flows through the cooling system countercurrently to the flow of synthesis gas through the catalyst bed. In the process, the synthesis gas successively flows through an inflow region, a hot spot region and an outward gas flow region. Cooling water is converted to steam by heat transfer in the outward gas flow region and hot spot region of the reactor, also referred to as vaporizer. Subsequently the steam is being superheated in the hot spot region of the reactor, also referred to as superheater. An external heat exchanger is used to preheat the water before entering the reactor against the effluent methane/synthesis gas mixture from the reactor.
In the embodiment of FIG. 2 of U.S. Pat. No. 4,839,391 a steam chamber is interposed between the vaporizer and the superheater. The steam flowing from the vaporizer collects in the steam chamber where still unvaporized cooling water carried along with the steam is separated. Collected dry steam is led to the superheater tube system and converted.
In the embodiment of FIG. 8 of U.S. Pat. No. 4,839,391 the vaporizer of the cooling system stretches over the entire length of a catalyst bed including the outward gas flow region, the hot spot region and the gas inflow region of a methanation reactor. A superheater is disposed in the hot spot region but projects into the outward gas flow region such that the superheater and vaporizer overlap for the full length of the superheater. A design is described that comprises steam superheating piping within catalyst tubes containing the catalyst, such that the reaction heat can be passed to both the coolant that is to be vaporized which surrounds the catalyst tubes and also to the vapor that is to be superheated.
The tube walls separating the superheated steam and the catalyst in the hot spot region need to be able to withstand high temperature and pressure. The temperature in the hot spot region and therefore the temperature of the tube walls separating the superheated steam and the catalyst may be more than 700° C. The tubes therefore require expensive construction material for its walls. In addition, the cooling system has a complex structure increasing the costs of the tube system even further.
In the embodiment of FIG. 6 of U.S. Pat. No. 4,839,391, the methanization reactor comprises a coolant vaporizer and a coolant superheater which form tube systems in the catalyst bed of the reactor. The reactor comprises a coolant preheater in the gas inflow region, a vaporizer exclusively located in the outward gas flow region of the methanation reactor and a hot spot region, which is cooled only by superheating steam. Although the reaction in the hot spot region is cooled by superheating the steam, the reaction temperature in the hot spot region is still very high. The temperature in the hot spot region and therefore the temperature of the reactor walls may be as high as 730° C. The reactor walls in the hot spot region therefore need to be able to withstand high temperature and pressure. In addition the reactor walls in the hot spot may require insulation. The hot spot region of the reactor therefore requires more expensive construction material for its walls than the other parts of the reactor.
U.S. Pat. No. 4,431,751 describes a method for producing superheated steam with the heat of catalytic methanation of a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen. The process passes a gas stream first through a first internally water-cooled reactor, subsequently through an adiabatic reactor and a subsequent heat exchanger and finally through a second internally water-cooled reactor. The internally water-cooled reactors comprise a cooling system disposed within a catalyst bed through which water flows. As catalyst a nickel-containing catalyst is used. Water is passed in succession, first through the cooling system of the second internally water-cooled reactor for preheating thereof to a temperature approximating the saturated steam temperature, thereafter into the first internally water-cooled reactor for conversion into saturated steam and subsequently to the heat exchanger following the adiabatic reactor for superheating.
A disadvantage of the process of U.S. Pat. No. 4,431,751 is that a separate adiabatic reactor is needed to superheat the steam.
It would be an advancement in the art to provide a cheap and economic process for the co-production of superheated steam and methane. It would further be a special advancement in the art to provide a cheap and economic process, which would neither require any adiabatic reactors nor any expensive construction materials for superheating the steam.